Traditionally, all crested gibbons have been considered
being members of a single species of the genus Hylobates (i.e. "H.
concolor"), and this species was considered the only representant of the
subgenus Nomascus. The yellow-cheeked crested gibbon has been considered being
a subspecies ("H. concolor gabriellae") of that species (e.g. Groves,
1972; Marshall, & Sugardjito, 1986). More recently, vocal characteristics and
other features suggested that yellow-cheeked crested gibbon should be recognized
as a distinct species (Geissmann, 1995a; Geissmann et al., 2000). In addition, recent
molecular evidence documented that the distance among gibbon subgenera was as large
or larger than the distance between chimpanzees (Pan) and humans (Homo)
(Roos & Geissmann, 2001). As a consequence of this finding, all four subgenera
are now recognized as full genera, and the traditional scientific name of the yellow-cheeked
crested gibbon changes from "Hylobates concolor gabriellae" to Nomascus
gabriellae.

No subspecies of the yellow-cheeked crested gibbon
are known, but the affinities of the gibbon populations in the northern part of the
distribution area unknown (Geissmann, 1995a; Geissmann et al., 2000).

Range

Habitat

This species is found in tropical evergreen forests.
They are scarce between 1,500 and 2,000 m (Eames & Robson, 1993) and appear to
prefer lowland forests (Dao Van Tien, 1983).

Ecology and diet

The yellow-cheeked crested gibbon remains virtually
unstudied in the wild. Like other gibbons, it is an arboreal and a diurnal species.
Like other gibbons, it probably prefers the upper canopy of the forest, and sleeps
and rests in emergent trees (Leighton, 1987). It is mainly frugivorous, but will
also consume leaves and insects.

Reproduction and ontogeny

Gibbons give birth to single offspring. The birth
interval in gibbons is about 2-3 years.

The gestation period in gibbons has a duration
of about 7 months, i.e. about 183-225 days (Geissmann, 1991).

Young gibbons are born with virtually hairless
ventral parts and must rely on their mothers for warmth.

Nursing lasts about two years.

Young gibbons will stay with their parents until
they are past adolescence.

In captivity, gibbons usually attain sexual maturity
at an age of about 6-8 years, but this appears to be highly variable (Geissmann,
1991).

At maturity, offspring probably leave the natal
group or may be chased off by their parents.

Life span: in captivity probably up to 50 years

Dimensions

Head and body length: about 60-80 cm (24-31 inches)
(estimate)

Body weight: 7.0-7.4 kg (15.4-16.3 lbs) (Geissmann,
unpublished)

Males and females are of about the same body size.

Morphology

Gibbons have no tail.

Gibbons have extremely long arms and relatively
long legs. The hands are also elongated and hook shaped. The long arms and hands
assist in suspensory locomotion.

The thumbs are not elongated and are not used for
swinging from branch to branch. They are opposable and are used for tactile probing
and grooming.

The body is small and typically held in an upright
position.

Locomotion

Gibbons typically exhibit a highly specialized form
of locomotion which is called brachiation. They swing below the branches suspended
by their arms. Brachiation is an energetically advantageous mode of locomotion. It
facilitates feeding in the fine branch niche. It allows for relatively high speeds
in the canopy and for jumps of 10 meters or more (Fleagle, 1999). When moving on
branches or on the ground, gibbons walk on two legs (bipedalism), often using their
arms for balance.

Coloration

The pelage of adult males is black with small pale
yellow or pale orange cheeks. Males also have a group of erect hairs (a crest) on
the top of their heads, thus the name "crested gibbons" for the genus Nomascus.

The pelage coloration of adult females strongly contrasts with that of adult males
(sexual dichromatism). Females have a bright yellow or pale orange pelage with a
black patch on the top of their heads.

When a yellow-cheeked crested gibbon is born, its coat is a bright yellow color.
During the second half of the first year, the color changes to black within a few
months; only the cheek patches remain yellow. The gibbons then resemble adult males
in their fur coloration. Females turn back to the yellow coloration about when attaining
sexual maturity. The timing of the color changes is variable and again takes several
months to completion.

Group size and social structure

Although yellow-cheeked crested gibbon are unstudied
in the wild, they probably live in small, monogamous family groups. Typical groups
probably consist of an adult pair with 0-4 immature offspring. Young gibbons leave
their natal group when they become adult.

Gibbons are territorial (Leighton, 1987). Each family
group occupies an area of about 20-50 hectares, but the typical territory size of
this species has not been reliably determined in the wild. Territories are defended
from intrusion by other gibbons by loud morning songs and by actively chasing intruders
off of the territory.

At night, gibbons sleep sitting up. The family group
spends the night in one of several preferred "sleeping" trees of the territory.

Vocal communication

Gibbon groups produce loud, stereotyped song bouts
in the early morning. Songs probably serve to defend resources such as territories,
food trees, partners, but may also help to attract potential mates. Gibbon songs
include species specific characteristics which are inherited and not learned (Geissmann,
1993).

Olfactory communication

Crested gibbons exhibit extended fields of skin
glands situated in the axillary, sternal and inguinal areas of the body. The glands
produce a reddish secretion and are particularly active under hot temperatures and
when the animals are exited. It has been speculated that the glands may play a role
in olfactory communication (Geissmann, 1993; Geissmann & Hulftegger, 1994). The
glandular secretion also influences the amount of red visible in the yellowish female
pelage coloration.

Tactile communication

Social grooming probably plays a role in reinforcing
the bonds between group members.

Psychology

There is some evidence for self-recognition in the
mirror (Ujhelyi et al., 2000).

Predators

Adult gibbons typically live in the crown region
of the forest where they have no natural predators except man. In the lower stories
of the forest, leopards, clouded leopards, and pythons may be potential predators
of gibbons.

Wild population estimates

Not available.

Status and conservation

The species is threatened by habitat loss (from
development and logging) and by hunting for food, for Oriental medicine and for the
pet market. Extensive military activities have also had a detrimental effect on the
species mainly through habitat destruction (Duckworth et al., 1995, 1999; Eames &
Robson, 1993; Geissmann, 1995b; Geissmann at al., 2000).

This species is not efficiently protected anywhere,
at present, not even in nature reserves and national parks (Geissmann, 1995b; Geissmann
at al., 2000).